This invention relates to horticultural structures for growing plants, seeds, cuttings and the like. The plant growth structures prepared according to the present invention provide a more optimal relationship between the water, oxygen, carbon dioxide, nitrogen, ethylene and other gases and plant roots. This improved relationship promotes better plant growth and reduces or eliminates the harmful effects due to what is commonly referred to as "over-watering." Over-watering prevents required amount of oxygen from reaching the roots and provides an environment conducive to the growth of certain pathological microorganisms.
It is well known in the art that artificial substrates can be used for the germination, rooting and propagation of horticultural crops. Substrates such as peat, vermiculite, perlite, wood bark, sawdust, certain types of fly ash, pumice, plastic particules, glass wool, organic and inorganic fibers, polyurethane, polystyrene, polyethylene, phenol formaldehyde and urea formaldehyde foams are now commonly used, or have been disclosed in the literature, either alone or in various admixtures with each other and/or soil. Furthermore, certain of these materials, alone or in various combinations, have been converted into rigid, self-supporting or expandable structures that are capable of supporting themselves and the plant without external confinement.
For example, U.S. Pat. No. 3,812,619 granted to Wood et al. on May 28, 1974 describes horticultural foam structures prepared by reacting an isocyanate capped polyoxyethylene polyol reactant with large amounts of an aqueous reactant which may contain seed and/or materials useful or necessary for plant growth. In addition, certain fibers can be converted with the aid of polymeric binders into plant growing structures.
In British Pat. No. 1,134,465, dated Nov. 27, 1968 there is described a plant growth medium molded from polymergrafted cellulose fibers which serves as the sole support and growth medium for propagation and growth of seedling plants and plants from cuttings.
Although these prior art structures are useful and have achieved commercial acceptance in many areas, they do not provide an optimal balance between water and the gases that can significantly influence root and total plant growth. Much experimental evidence has been collected to demonstrate that oxygen is necessary for plant root growth and respiration. The rate of diffusion of oxygen through a reasonably thick layer of water alone is not sufficient to maintain an optimum concentration for proper root growth. L. H. Stolzy and J. Letey have published threatises in (1) Soil Science, Vol. 103, No. 6, pages 404-409 in 1967, (2) Hilgardia, Vol. 35, No. 20, pages 567-576 in October 1964, and (3) Advances in Agronomy, Vol. 16, pages 249-279 in 1964 on the subject of oxygen diffusion rates (ODR) and to the extent necessary for an understanding of this invention such as incorporated herein by reference. The critical ODR value of a substrate below which roots of many plants will not grow is about 20 .times. 10.sup.-.sup.8 g cm.sup.-.sup.2 min.sup.-.sup.1. Clearly, with reduced root growth, foliage and other organ growth will also be retarded. ODR is obviously strongly influenced by air filled voids within the substrate. It is well known that plants growing in such commonly used substrates identified above can, under some conditions, suffer from lack of oxygen and show the symptoms commonly believed to be caused by over-watering, i.e., chlorosis, slow growth, pale color, and even death.
It is the object of the present invention to produce a substrate which eliminates or minimizes the plant growth problems mentioned above. It has been found that plant growth in a substrate such as a synthetic foam, peat or fiber block, pumice and the like can be drastically improved by introducing a plurality of holes or conduits, 1-5 in nominal diameter, extending throughout the substrate, spaced about 1-8 mm, preferably 2-4 mm apart. The conduits will drain water from the substrate and provide reservoirs of oxygen for the plant roots and at the same time the substrate will hold water around the conduits which will be available to the roots. By way of example, a substrate is provided with conduits and placed in a conventional pot. Soil or the like is deposited on top of the substrate and a seed, cutting or small plant is placed in the soil. With the substrate in the pot, over-watering induced problems are virtually prevented. Any excess water will drain from the substrate and out of the opening in the bottom of the pot. Additionally, since water drains from the substrate the conduits will be filled with air and oxygen will be readily available to the roots.
Thus, the invention can be described as a plant growth medium or substrate, capable of absorbing water (and plant nutrients ) and containing a plurality of holes, 1-8 mm apart, preferably, but not limited to, holes 2-4 mm apart and about 1-5 mm in diameter, preferably, but not limited to 1-3 mm in diameter. The basic medium or substrate can be composed of synthetic foams such as polyurethane, phenol formaldehyde, urea formaldehyde, polystyrene and the like, fiber structures composed of peat, wood fiber, other natural and synthetic fibers with or without binders, inorganic foams such as pumice, certain types of fly ash, foamed glasses and the like. Various combinations of the above can also function in the described invention.
It is also within the scope of the present invention to include materials such as soils, clay, vermiculite, perlite, bark wood shavings or chips, ion exchange materials, and the like in the preparation of the basic substrate. It is also possible to use particles of these various materials that are bound together in some fashion.
It is further within the scope of the present invention to use a substrate which has a major part of its external surface coated with a film that retards the evaporation of water from the coated surfaces. Such fluid impermeable films are well known and need not be described for an understanding of the invention. Ordinarily, foraminous bodies confined in a container with an overlay of soil would not require a coating but where the substrate stands alone, without soil overlay or surrounding container, the coating may be desirable, but is not absolutely necessary.
It is an object of this invention to produce a plant growth substrate that, upon the addition of water and appropriate nutrients, can be used for the germination of seeds and growth of seedlings, the vegetative propagation and growth of plant material and the growth of plants to maturity or some other stage of growth and development.
It is a further object of this invention to provide a substrate that can be beneficially used in combination with the normal soils, soil mixtures and synthetic medias, such as peat, vermiculite, etc. In particular the hole containing substrates can be used to replace a substantial amount of the soil media in a conventional horticultural container, substantially improve the total water holding ability of the soil media-substrate combination and maintain at the same time a more optimal balance between solids, water and gases.
It is still a further object of this invention to provide a propagation block that allows for more rapid penetration and more homogeneous growth of roots throughout the volume of the propagation block. These roots are consequently better able to use the water contained in the block and thus reduce the frequency of watering and make care of the plant easier. In addition the more homogeneous distribution of roots and their more rapid penetration of the block permits more rapid growth of roots into the soil media after the plants are transplanted. The combination of better water utilization and more rapid growth into the soil reduces problems associated with the commonly observed transplant shock phenomenon.